The present invention is directed to a CCD sensor.
An apparatus capable of acquiring two-dimensional information by employing a CCD is known from, for instance, European Patent Publication EP-0881486 entitled “Method For Measuring Physical Phenomenon, or Chemical Phenomenon, and Equipment Thereof”. As a sensor used in this measuring apparatus, such a CCD sensor is employed which is arranged by a plurality of physical/chemical sensors (unit sensors) sensible to physical/chemical phenomena, and also, a plurality of CCDs which transfer electric signal charges produced from these plural unit sensors.
FIG. 3 is a diagram for schematically indicating a structure of an upper surface of a CCD sensor which measures, for example, a two-dimensional distributing condition of pH. In this drawing, reference numeral 1 indicates a CCD sensor. This CCD sensor 1 is constituted by a plurality of unit elements (unit sensors) 2 sensible to hydrogen ions. While the upper surfaces of the respective unit elements 2 are covered by pH responsive films capable of sensing (responding) with respect to hydrogen ions corresponding to a measuring object, potentials are produced at boundary surfaces between the pH response film and a liquid in response to an amount of hydrogen-ion concentration, and a magnitude (namely, pH) of the hydrogen ion concentration is converted into an electric charge signal by utilizing a change of this potential. While, for example, 10 pieces of these unit sensors 2 are provided along a direction indicated by an arrow 4 so as to constitute a sensor column 3, for instance, 5 sets of these sensor columns 3 are arrayed along a direction indicated by an arrow 5 which is located perpendicular to the above-described arrow 4 so as to arrange the CCD sensor.
Reference numeral 6 shows a charge transfer portion for transferring electric charge signals produced in the respective unit sensors 2. This charge transfer portion 6 is arrayed by a plurality of vertical CCDs 7 and a single horizontal CCD 8, while the vertical CCDs 7 are arranged by a plurality of CCDs. Also, reference numeral 9 represents a signal output portion for outputting a signal derived from the horizontal CCD 8. Reference numeral 20 represents a diffusion portion of the signal output portion 9.
FIG. 4 illustratively shows a basic structure of the above-explained CCD sensor 1. In this drawing, reference numeral 10 indicates a semiconductor substrate made of, for instance, p type Si (silicon), the thickness of which is selected to be on the order of 500 micrometers.
A channel stopper 11, a charge supplying portion 12, a charge injection control portion 13, a sensor portion 14 functioning as a charge converting portion, a barrier portion 15, a CCD 6a which constitutes the charge transfer portion 6, a floating diffusion 16, a reset gate 17, a reset drain 18, and an output transistor 19 having an MOS structure are formed on the above-described semiconductor substrate 10.
Then, a single piece of unit sensor 2 is constituted by the respective members such as the charge supply portion 12, the charge injection control portion 13, the sensor portion 14, and the barrier portion 15. As will be later discussed more in detail, the sensor portion 14 constructs such a potential well which is constituted in such a manner that a depth of this potential well is changed in response to a magnitude of hydrogen-ion concentration. Also, the signal output portion 9 is constituted by the respective members such as the floating diffusion 16, the reset gate 17, the reset drain 18, and the output transistor 19.
Now, a principle idea of measuring operations of the above-described unit sensor 2 will be explained with reference to a potential diagram indicated in FIG. 5. When the measuring operation is carried out, a pulse voltage is applied to the charge supplying portion 12, the barrier portion 15, and the reset gate 17, whereas a DC voltage is applied to other electrodes except for the floating diffusion 16.
On the other hand, in an MOS structure using a p type semiconductor, the following fact is normally known. That is, since a positive voltage is applied to a metal electrode of the MOS structure, a depletion layer is formed on a boundary surface between an insulating film and the semiconductor in response to this positive voltage. As a consequence, while this phenomenon is used, potential states are produced in the vicinity of the boundary surface between the semiconductor and the insulating film, as represented in FIG. 5.
In a state as indicated in FIG. 5A, the potential of the charge supplying portion 12 is set to a high potential (namely, arrow direction is high potential), and electric charges 21 are not injected into the sensor portion 14.
In a state as indicated in FIG. 5B, since the potential of the charge supplying unit 12 is decreased, the electric charges 21 are injected into the sensor portion 14.
In a state as indicated in FIG. 5C, since the potential of the charge supplying unit 12 is increased, the electric charges 21 which have been cut off by the charge injection control portion 13 are stored into the sensor portion 14.
In a state as shown in FIG. 5D, since the potential of the barrier portion 15 is increased, the electric charges 21 which have been stored in the sensor portion 14 are transferred to the floating diffusion 16.
In a state as shown in FIG. 5E, after all of the electric charges 21 of the sensor portion 14 have been transferred to the floating diffusion 16, the barrier portion 15 is closed so as to stop the flow-in operation of the charges. At this stage, the potential of the floating diffusion 16 is determined based upon an amount of such transferred electric charges 21, this potential is inputted to the gate portion of the output transistor 19 having the MOS structure, and a drain current of this output transistor 19 is measured by a source follower circuit.
In a state as indicated in FIG. 5F, after the potential of the floating diffusion 16 has been read, the reset gate 17 is turned ON so as to reset the potential of the floating diffusion to the potential of the reset drain 18. Since this reset operation is performed, the potential state is again returned to the above-explained state shown in FIG. 5A. In other words, since the operations defined from the state of FIG. 5A to the state of FIG. 5F are repeatedly carried out, the electric charges can be derived outside the MOS structure.
In accordance with the CCD sensor having the above-described structure, phenomena occurred at a plurality of different positions can be measured at the same time. Since the magnitude of the hydrogen ion concentration is converted into the electric charges, either a one-dimensional distribution of pH or a two-dimensional distribution of pH can be easily acquired as an image by employing the charge transfer portion 6 constituted by a plurality of CCDs 6a. Furthermore, electric charges indicative of information as to plural points are stored, so that a very weak signal can be amplified. Accordingly, a very small change occurred in a phenomenon may also be firmly grasped.
As previously explained, in such a case that the two-dimensional information is obtained by employing the CCD, the electric charges appeared in the detecting elements 2 arranged in the two-dimensional manner are continuously carried to the diffusion portion 20 of the signal output portion 9, and then, are converted into the voltage by this diffusion portion 20. This voltage may be outputted outside this CCD. In such a case that a chemical amount such as ion concentration, or a physical amount such as a temperature, which are wanted to be measured, constitutes only a portion of an entire amount, and furthermore, a signal amount is small, S/N (signal-to-noise ratio) is necessarily determined based upon performance of an FET of the above-explained diffusion portion 20.
Under such a circumstance, the following idea may be conceived. That is, the areas of the respective detecting elements 2 may be increased so as to carry a large amount of electric charges. In this case, while potential inclinations (fringe effect) at the detecting elements 2 are not involved, there is a limitation in increasing of the above-described areas of these detecting elements 2. As a result, it is practically difficult to achieve sufficiently high S/N.